1. Field of the Invention
This invention relates to the production of rubber-modified monovinyl aromatic polymers, and more particularly, to a method and apparatus for making high impact polystyrene (HIPS) by continuously polymerizing butadiene, exchanging the hydrocarbon solvent used in the butadiene polymerization with styrene, and feeding the resultant polybutadiene/styrene stream to a conventional HIPS production line.
2. Description of Related Art
Methods and apparatus for continuously producing polybutadiene and for producing high impact polystyrene using polybutadiene as the rubber component, are both well known. The solution polymerization of butadiene to polybutadiene in a hydrocarbon solvent is disclosed, for example, in U.S. Pat Nos. 4,271,060; 4,375,524; 4,495,028; and 4,686,086, the written descriptions of which are incorporated herein by reference. According to conventional methods, polybutadiene is made by polymerizing butadiene to about 12 weight percent solids in hexane, butane, cyclopentane, cyclohexane, or another hydrocarbon solvent; concentrating the mixture to about 30 weight percent solids by flashing off solvent; steam stripping with soap to remove additional solvent, reduce stickiness and precipitate the crumb rubber; squeezing and drying to remove excess moisture; and agglomerating the dried crumb rubber by the addition of heat to produce irregularly shaped bales. The baled rubber is then sent to intermediate storage or transported to plant sites for use in making other products such as rubber-modified polymers.
One widely used rubber-modified polymer is high impact polystyrene (HIPS). HIPS is made by polymerizing styrene monomer having dissolved in it from about 1 to about 15 percent by weight styrene-butadiene rubber (SBR) or polybutadiene rubber. Both are commonly produced with Mooney viscosities of either 35 or 55, and polybutadiene is generally less expensive than SBR. A conventional method for making HIPS using polybutadiene is disclosed, for example, in U.S. Pat. No. 4,777,210 to Sosa and Nichols, the entire written description of which is incorporated herein by reference.
When bales of polybutadiene made by conventional, prior art methods are used in HIPS production, they are segmented or chopped and ground, then dissolved in a solvent/styrene mixture prior to initiating polymerization of the styrene monomer. Because the labor, equipment, transportation, storage and energy costs associated with finishing the polybutadiene bales and later grinding and dissolving the rubber for use in making HIPS are significant and desirably avoided if possible, a method and apparatus are needed for continuously producing and supplying a polybutadiene/styrene stream to a conventional HIPS production line.